(a) Field of the Invention
The present invention relates to an apparatus for cryogenic treatment, such as air distillation, of the type comprising a cryogenic treatment column and a heat exchange line to cool at least one fluid to be treated before its introduction into the column.
(b) Description of Prior Art
The operation of air distillation apparatuses requires heat insulating the cold parts at very low temperature which are essentially heat exchange line, the distillation column and the various "cold" accessories such as the valves and possibly liquid oxygen or liquid nitrogen pumps and/or expansion turbines.
Actually, insulation under vacuum of the apparatus as a whole is by far the most thermically performing technique. Unfortunately, its cost is high and its possibilities of use are limited, such as for the following reasons.
(1) For reasons of accessibility, the cold accessories as a whole mentioned above should be placed outside a sheath under vacuum, which requires a number of inlets/outlets ducts on this sheath.
(2) A sheath under vacuum, which by definition is cylindrical, is poorly adaptable for placing a distillation column therein, which is often quite high, and also one or more parallelipedic heat exchangers of the type with brazed plates. The result is a limitation on the sizes of the apparatuses which can be disposed in the sheath yet still permit easy transportability.
For these reasons, insulation under vacuum has been limited to apparatuses of small sizes, i.e. capable of treating a flow of air of the order of a few thousands of m.sup.3 /h, such as those which are kept cold by injection into the column of a cryogenic liquid of exterior origin, and which is relatively costly (so-called liquid assist apparatuses).
The invention aims at providing a heat insulation technique for apparatuses of cryogenic treatment in which the performances are close to an insulation under vacuum of the apparatus as a whole in spite of a substantially reduced cost, and which can be used with apparatuses of relatively large size while enabling easy transportation.